Effect of Neofabraea alba on bark and wood anatomy of Fraxinus spp.

Abstract

The anatomy of an unusual canker on Fraxinus spp. was investigated with elastomer microcasts. The canker was caused by a new fungal disease, 'Coin Canker of Ash', affecting nursery stock in Northeastern North America. Cankers on bark surfaces were remarkably round and copper-colored. Diseased areas of stems had non-parallel orientations of axial parenchyma tissue as well as of the rays, vessel elements and fibers in contrast to healthy areas. Bark in diseased areas of stems was eroded beneath the surface and a callus was produced along the margins of damage that filled the cavity of eroded tissue. Diseased areas had large aggregates of sclereids compared to healthy areas. Ray initials in diseased areas of the stem were shorter, multiseriate and 3-10 or more cells in width compared to the longer uni- and biseriate initials in healthy areas. Wood in diseased areas had circular vessels in tangential view due to a change in the shapes of individual vessel elements, compared to parallel and straight vessels in healthy areas. Individual elements became spindle-shaped and gave rise to zigzag vessels. The fungal pathogen, Neofabraea alba, appeared to alter the way in which cambial cells differentiated.